In the integrated circuit (IC) industry, integrated circuits need square-wave repeating clock signals to execute software and perform other useful operations. To enable creation of these clock signals, integrated circuits have external terminals which are connected to crystal oscillator circuitry. The crystal oscillator circuitry typically contains a crystal oscillator component along with one or more passive elements such as capacitors, resistors, and the like. This crystal oscillator circuit provides the sinusoidal repeating signals to the IC so that IC square-wave clocks may be created and routed to all functional circuitry within the IC. In order to charge the oscillator circuit's capacitors and start the crystal oscillator, an electrical signal (e.g., current) is typically communicated from the integrated circuit to the external oscillator circuitry. Due to the inherent nature of crystal oscillators and the capacitors they are coupled to during operation, stable crystal oscillation and clock generation consumes a significant amount of time during the "start up" procedure of an IC.
In the prior art, a current source internal to the integrated circuit which powers the external oscillator circuitry during "start up" is statically configured. In one form of this prior art embodiment, the internal current source is statically configured to provide a high current all the time (i.e., during both the start up period of the integrated circuit and during the IC's normal mode of operation). When this current source is providing high current constantly, the start up time of the integrated circuit is minimized or rendered optimal since the higher current powers the external oscillator circuit at a faster rate. However, the electromagnetic interference (EMI) is typically too high when ICs use static high oscillator current whereby electrical operation of adjacent circuitry is impeded.
In another form, the internal current source of the integrated circuit may be statically set to provide low current at all times. When using this low-current design, the integrated circuit has reduced electromagnetic interference (EMI), but will typically have a lengthy start up time which is unacceptable in most systems. Therefore, a statically configured high current source and/or a statically configured low current source for use with oscillator circuitry is disadvantageous for most modern applications.
In order to overcome the above disadvantages, integrated circuits are now being designed with automatic gain control (AGC) circuitry of circuitry that is also referred to as automatic level control (ALC) circuitry. This AGC and/or ALC circuitry is complex analog circuitry that replaces the statically configured current source discussed previously. The AGC or ALC is an analog circuit that monitors the amplitude of the oscillator signal and continuously adjusts the current of the oscillator amplifier until an optimal oscillator amplitude is obtained for a given IC device. The AGC/ALC circuitry does not have discrete and static current levels as does the prior art, and changes current levels based on amplitude of the oscillator signal. The AGC or ALC circuitry will consume a significant size of the integrated circuit and is a costly circuit to design and manufacture. In addition, since the AGC and/or ALC circuitry is analog in nature, environmental and process variations may affect the performance and operation of these circuits. Most AGC/ALC circuits are not process scalable. While these AGC/ALC circuits are complex, more expensive, not very scalable, and consume a significant silicon surface area, these circuits allow for continuous current control of the oscillator circuitry so that start up time is somewhat improved while EMI effects are reduced.
Therefore, a need exists in the industry for an improved clock generation circuit which reduces EMI effects and improves start up time, while simultaneously providing a less complex solution which consumes less surface area and is more cost effective than previous analog solutions.